Alkaline pretreatment is one of major chemical pretreatment technologies receiving numerous studies. It employs various bases, including sodium hydroxide (NaOH) [83], calcium hydroxide (lime) [84], potassium hydroxide (KOH) [85], aqueous ammonia [86], ammonia hydroxide [87], and NaOH in combination with hydrogen peroxide or others [88-90]. Among these alkaline pretreatments, lime has received much more attentions since it is inexpensive (about 6 % cost of NaOH), has improved handling, and can be recovered easily by using carbonated wash water [91].

8.3.2.1 Process Description

In comparison with other pretreatment technologies, alkali pretreatment usually uses lower temperatures and pressures and even ambient conditions. Pretreatment time, however, is recorded in terms of hours or days which are much longer than other pretreatment processes. In the alkaline pretreatment, the residual alkali could be reused through the chemical recycle/recovery process, which may make the system more complex due to the need for chemical recovery [92, 93]. The particle size of the biomass is typically 10 mm or less [57]. A significant disadvantage of alkaline pre­treatment is the conversion of alkali into irrecoverable salts and/or the incorporation of salts into the biomass during the pretreatment reactions so that the treatment of a large amount of salts becomes a challenging issue for alkaline pretreatment [92]. The effectiveness of alkaline pretreatment varies, depending on the substrate and treatment conditions. In general, alkaline pretreatment is more effective on hard­wood, herbaceous crops, and agricultural residues with low lignin content than on softwood with high lignin content [94]. In addition, in comparison with KOH and lime, pretreatment with NaOH was found to be more efficient for the subsequent enzymatic hydrolysis [92].